Optimum Dietary Lipid Requirement And Short-Chain Fatty Acids Nutritional Function For Juvenile Golden Pompano (Trachinotus Ovatus)

Trachinotus ovatus,also known as golden pompano,is characterized by rapid growth,euryhalinity and easy domestication,which is one of the important economic fish in China.As an important nutrient,lipids can provide essential fatty acids,phospholipids,sterols and lipid-soluble vitamins,which play an important nutritional function for T.ovatus.Insufficient or excessive dietary lipid can adversely affect fish,so exploring the optimal lipid requirement has positive influence for the healthy aquaculture of T.ovatus.The shortage and rising prices of traditional dietary protein such as fish meal led to higher feed costs.Searching new functional additives is important for reducing feed cost and developing environment-friendly feed.In this study,we analyzed the growth performance,serum biochemical and immune index,antioxidant capacity and expression levels of liver lipid metabolism-related genes by nutriology,immunology and molecular biology,and obtained the optimal dietary lipid level for T.ovatus.Meanwhile,the nutritional functions of short-chain fatty acids(formic acid,acetic acid and propionic acid)were studied by using microbiomics,metabolomics and transcriptomics.The main research results are as follows:1.The optimal dietary lipid level for T.ovatus.Five isonitrogenous diets were formulated containing five graded levels of lipid(52.0,81.0,112.0,141.0 and 171.0g/kg).The diets were fed to juvenile T.ovatus(10.49±0.23g)for 8 weeks.Results showed that 112.0g/kg lipid level in diets significantly increased weight gain rate,specific growth rate and feed efficiency rate(P<0.05).The dietary lipid level affected plasma biochemical,activities of plasma total lipase,alanine aminotransferase and aspartate aminotransferase,and activities of hepatic total lipase and fatty acid synthetase as well as activities of intestinal lipase,adipose triglyceride lipase and hormone-sensitive lipase(P<0.05).The relative level of hepatic carnitine palmitoyl transferase I m RNA increased significantly with increasing dietary lipid levels.The m RNA level of hepatic fatty acid synthase was significantly upregulated with dietary lipid level up to 81.0g/kg and then decreased(P<0.05).Moreover,dietary lipid level influenced microbial community to regulate metabolic capacity of intestine.The optimum dietary lipid level for the optimal growth of juvenile T.ovatus was 93.6g/kg.2.The optimal supplement levels of three short-chain fatty acids for juvenile golden pompano diets.(1)Potassium diformate(KDF)Six diets with 0,3.00,6.00,9.00,12.00 and 15.00g/kg potassium diformate were formulated to feed golden pompano(5.87±0.03g)for 8 weeks.The results indicated that6.00 and 9.00g/kg KDF significantly increased growth performance,reduced feed conversion ratio and improved fillet quality and intestine morphology of juvenile T.ovatus(P<0.05).Dietary 9.00 g/kg KDF significantly decreased triglyceride and glucose of plasma and increased high density lipoprotein cholesterol(P<0.05).6.00g/kg KDF significantly up-regulated m RNA levels of Nrf2,CAT,CuZn-SOD,MnSOD and increased antioxidative enzyme activities in liver(P<0.05).Furthermore,6.00g/kg KDF significantly enhanced immune function of juvenile golden pompano by regulating the expression levels of hepatic NF-κB,TGF-β,ZO-1,Occludin and Claudin-3a(P < 0.05).The optimum supplement level of KDF in golden pompano diet was 6.58g/kg based on WGR.(2)Sodium acetate(SA)Six isonitrongenous and isolipidic diets were formulated to feed fish(6.06±0.02g)with graded levels of sodium acetate(0,250,500,1000,2000,4000mg/kg)for 8 weeks.The results revealed that 1000 mg/kg SA significantly improved growth performance,improved fillet quality,increased enzyme activities of hepatic superoxide dismutase and total antioxidant capacity and decreased malondialdehyde contents as well as downregulating the expression levels of Keap1,IL-8,TNF-α and NF-κB genes and upregulating the expression levels of Nrf2,TGF-β genes of spleen(P<0.05).Supplement levels of dietary SA significantly influenced plasma biochemical parameters containing triglyceride,glucose,high density lipoprotein cholesterol,complements and immunoglobulin M(P < 0.05).Excess SA caused negative effects for T.ovatus.The optimum supplement level of SA for juvenile golden pompano was evaluated to be 1423.67 mg/kg based on WGR.(3)Sodium propionate(PA)Seven isonitrongenous and isolipidic diets were formulated to feed fish(8.64±0.08g)with graded levels of sodium propionate(0,2.00,4.00,6.00,8.00,10.00,12.00g/kg)for 8 weeks.The results revealed that 4.00 and 6.00g/kg SP significantly promoted weight gain rate,specific growth rate and feed intake,significantly increased high density lipoprotein cholesterol and complement 3 of plasma(P<0.05).Furthermore,4.00 and 6.00g/kg SP significantly decreased viscerosomatic index and hepatosomatic index,plasma triglyceride and glucose contents and decreased hepatic malondialdehyde contents(P<0.05).6.00 and 8.00g/kg SP level significantly improved lipid contents and decreased protein contents and hardness of muscle(P<0.05).Dietary 6.00g/kg SP significantly influenced hematological parameters,intestinal digestive enzymes activities and morphology as well as significantly regulating the expression levels of intestinal immune-related genes TGF-β,IL-8,TNF-α,IL-10,ZO-1,Occludin,Claudins and HSP70 for T.ovatus(P<0.05).The optimum supplement level of SP for juvenile golden pompano was evaluated to be 6.02g/kg based on WGR.3.The effects of three optimal supplement level of SCFAs diets on the intestinal health of golden pompano.Based on the optimal supplement level of KDF,SA and PA,four isonitrogen and isolipidic diets were formulated for the control group(C)and the experimental groups(FA,SA and PA)and then fed juvenile golden pompano(8.66±0.06g)for eight weeks.The results indicated that FA,SA and PA diets significantly changed SCFAs of intestinal contents(P<0.05).FA,SA and PA diets increased abundance of Tenericutes,Actinobacteria,Cyanobacteria and Firmicutes and reduced the abundance of Proteobacteria.Alpha diversity of intestinal microbiota was significantly improved in the FA,SA and PA groups(P<0.05).Compared with the C group,the groups of FA,SA and PA increased the abundance of beneficial bacteria such as oscillospira,turicibacter,and synechococcus and decreased the abundance of harmful bacterial genera such as photobacterium.A total of 4,985 lipids were identified between the C and FA group by lipidome analysis.Compared with C treatment,the FA treatment identified 286 upregulated and 280 downregulated lipids.4,987 lipids were identified between the C and SA group by lipidome analysis and there were 297 upregulated and 290 downregulated lipids in the SA treatment.4,987 lipids were identified between the C and PA group by lipidome analysis and there were 242 upregulated and 265 downregulated lipids in the PA treatment.In FA,SA and PA groups,glycerides(TG,MG and DG)of intestinal contents were decreased.Glycerophospholipids,such as PC,PE,LPE and LPA were increased,while PG、PS、PI and LPS were decreased in FA,SA and PA groups.Sphingolipids,such as SM was decreased and Cer and ST were increased in FA,SA and PA groups.In addition,the Co-Q10 coenzyme and DHA contents in the FA and PA groups were also increased.The transcriptome analysis revealed a total of 5,705 differential genes,with 3,100 upregulated genes and 2,605 downregulated genes."Immune system process" and "transcription regulator activity","PI3K-Akt signaling pathway","Endocytosis","AMPK signaling pathway" and "Tight junctions" were significantly upregulated in the SA group.4.The effect of optimal supplement level of SA diets on the liver metabolism.Based on the optimal supplement level of SA,two isonitrogen and isolipidic diets were formulated for the control group(C)and the experimental groups(SA)and then fed juvenile golden pompano(8.66±0.06g)for eight weeks.The results showed the expression levels of the lipid metabolism-related genes PPAR,cpt1,TOR,SREBP-1,FAS,LPL,and HSL were all significantly upregulated in the liver of T.ovatus fed SA diets.A total of 4,816 differential genes were identified in groups C and SA,with 2,510 upregulated genes and 2,306 downregulated genes.The supplement of SA mainly involved the upregulation of "regulation of immune system process","defense response","biosynthesis of secondary metabolites" and "complement and coagulation cascades".A total of 182 differential lipids were identified between the two groups,which included 69 upregulated and 113 downregulated lipids.Glycerophospholipids,such as PC and PE were decreased,while PS,PI,LPE,and LPC were increased;TG in SA group were increased.Sphingolipids,such as SM and SPH were decreased and Cer and ST were increased.